Recently, a non-hear treated steel to be hot-forged (hereinafter, referred to as a non-heat treated steel) in which heat treatments can be omitted is applied to a forged component for a vehicle engine and a forged component for a vehicle suspension. The chemical composition of the non-heat treated steel is designed to realize excellent mechanical properties even when the non-heat treated steel is as air-cooled or as forced-air-cooled after hot forging, that is, even when the heat treatments including quenching and tempering according to the related art are omitted.
As one of the components to which the non-heat treated steel is widely applied, there is a connecting rod (hereinafter, called a conrod) for an engine. The conrod is a component which converts a reciprocating motion of a piston in the engine into a rotating motion of a crankshaft to transmit power and is composed of two pans, i.e. a cap and a rod. The conrod is mounted to the crankshaft by interposing the crankshaft between the cap and the rod and fastening them with bolts. Hitherto, the conrod is manufactured by separately forging the cap and the rod or mechanically cutting a product forged to have a shape in which the cap and the rod are integrated, and thereafter processing joint surfaces of the cap and the rod by machining with high accuracy. In addition, in many cases, pin-cutting is performed to prevent the joint surfaces from misalignment. Therefore, the processing becomes more complex, and thus there is a problem in that the manufacturing costs are increased.
Therefore, in recent years, a method including hot forging a steel to form the steel into a shape in which a cap and a rod are integrated, notching the inside of a large end portion of the formed product, cold fracture splitting the formed product into the cap and the rod by applying an impact tensile stress to the formed product, and mounting the cap and the rod to a crankshaft using the fractured surfaces thereof as joint surfaces is employed. In this method, machining of the joint surfaces is unnecessary. In addition, pin-cutting to prevent misalignment can be omitted as necessary using irregularities of the fractured surfaces. Therefore, the processing cost of components can be reduced. Moreover, since the area of the joint surfaces can be reduced by omitting pins, it is possible to achieve reductions in the size and weight of the conrod itself.
In Europe and USA where such fracture split conrod is widely supplied, C70S6 in the DIN standards is supplied as a steel for the fracture split conrods. This is a high carbon non-heat treated steel containing 0.7 weight % of carbon, and in order to suppress changes in dimensions during fracture splitting, almost the entire structure thereof is a pearlite structure having low ductility and low toughness. An amount of plastic deformation of C70S6 in the vicinity of a fractured surface at the time of fracture is small and thus C70S6 has excellent fracture separability. On the other hand, C70S6 has a coarse structure compared to a ferrite-pearlite structure of a medium carbon non-heat treated steel which is a current steel for a conrod and thus has a low yield ratio (i.e. yield strength/tensile strength). Therefore, there is a problem in that C70S6 cannot be applied to a high strength conrod which requires high buckling strength.
In order to increase the yield ratio, it is necessary to control an amount of carbon low and to increase a ferrite fraction. However, when the ferrite fraction is increased, ductility and toughness are enhanced, and thus the amount of plastic deformation in the vicinity of the fractured surface during fracture splitting is increased, resulting in an increase in the amount of deformation of the inside diameter of the large end portion of the conrod. Therefore, there is a problem in that the fracture separability is degraded.
In order to solve the problems, a medium carbon non-heat treated steels having excellent fracture separability are proposed. For example, in Patent Documents 1 and 2, a technique of adding a large amount of an embrittling element such as Si or P in order to degrade ductility and toughness of a material itself so as to improve fracture separability is described. In Patent Documents 3 and 4, a technique of degrading the ductility and toughness of ferrite using precipitation strengthening of second phase particles so as to improve fracture separability is described. In Patent Documents 5 to 8, a technique of controlling the form of Mn sulfides so as to improve fracture separability is described. In Patent Document 9, a technique of heating a steel to an ultra-high temperature close to a solidus line or a liquidus line in order to significantly coarsen the structure of the steel so as to improve fracture separability is described. However, in the techniques, while the amount of deformation of the fractured surface obtained by the fracture splitting is small, the material becomes brittle, and thus chipping occurs during fracture splitting or during engaging the fractured surfaces with each other. Chips of the fractured surfaces cause misalignment in a position during the engagement between the fractured surfaces, and thus there may be a problem in which the engagement cannot be performed with high-accuracy.